Light pipe, surface light source device and reflection type liquid-crystal display device

ABSTRACT

A light pipe in which light incident on an incident side surface ( 13 ) is outputted from a lower surface ( 12 ) through a light output means formed on an upper surface ( 11 ), the direction of the largest intensity of the light outputted from the lower surface is within an angle of 30 degrees to a normal line with respect to a reference plane of the lower surface, the largest intensity of light leaking from the upper surface in the direction within the angle of 30 degrees is not higher than 1/5 as high as the largest intensity at the lower surface, and the light pipe has a hard coat layer-on the upper surface so that light incident on the lower surface is transmitted from the upper surface; a surface light source device having a light source disposed on the incident side surface of the light pipe; and a reflection type liquid-crystal display device having a liquid-crystal cell provided with a reflection layer disposed on the lower surface side of the surface light source.

TECHNICAL FIELD

The present invention relates to a light pipe adapted to form areflection type liquid-crystal display device which is bright and easyto see, and a surface light source device which uses the light pipe andwhich is excellent in effective utilizing efficiency of light.

BACKGROUND ART

An illumination device permitting a reflection type liquid-crystaldisplay device to be viewed in a dark place, or the like, has beenrequired. In the meantime, the inventors of the present invention havetried to use a front-lighting system in which a back-lighting unit usedin a transmission type liquid-crystal display device is disposed on thevisual side of a liquid-crystal cell. Such a back-lighting unit uses alight pipe for outputting light incident on its side surface from one ofits upper and lower surfaces through a light output means. In thefront-lighting system, display contents are recognized visually throughthe light pipe.

In the back-lighting unit using the background-art light pipe, therewas, however, a problem that it was difficult to put the back-lightingunit into practical use because of occurrence of shortage of contrast inthe on-state of the back-lighting unit and occurrence of shortage ofbrightness, disorder of display, etc. in the off-state of theback-lighting unit. Incidentally, in a back-lighting unit using a lightpipe having diffusing dots or fine irregularities as a light outputmeans, there arose a problem that contrast was short in visualrecognition in the on-state of the back-lighting unit whereas contrastand brightness of display were short as well as clarity was shortbecause of remarkable disorder of a display image through the light pipein visual recognition owing to external light such as room illumination,or the like, in the off-state of the back-lighting unit.

On the other hand, also in a back-lighting unit using a light pipe(JP-A-62-73206) having a stairstep prism structure including inclinedsurfaces at an inclination angle of 45degrees and flat surfaces at aninclination angle of 0 degrees as a light output means, there arose aproblem that contrast and brightness of display were short in visualrecognition in the on-state of the back-lighting unit.

Further, in the aforementioned front-lighting system, the light pipe waslocated as a surface on the visual side. When the surface was injured,the injury caused disorder of a display image in the same manner as inthe fine irregularities. Particularly when the light output means in thelight pipe was located in the surface and it was injured, leaking lightwas apt to increase or the light output means was apt to generate abright or dark portion. Moreover, when the light output means wasabraded gradually by wiping-out, or the like, there arose a problem thatthe light output characteristic of the light output means varied widely.

Therefore, an object of the present invention is to develop a reflectiontype liquid-crystal display device which is excellent in contrast andbrightness of display in visual recognition regardless of on-state andoff-state of the lighting unit, which is excellent in clarity because ofprevention disorder of a display image through a light pipe and which isexcellent in preservation of initial light output characteristic ordisplay characteristic, and develop a light pipe and a surface lightsource device that can form the reflection type liquid-crystal displaydevice.

DISCLOSURE OF THE INVENTION

According to the present invention, there is provided a light pipe inwhich: light incident on an incident side surface of said light pipe isoutputted from a lower surface of said light pipe through a light outputmeans formed on an upper surface of said light pipe; the direction ofthe largest intensity of the light outputted from said lower surface iswithin an angle of 30 degrees to a normal line with respect to areference plane of said lower surface; the largest intensity of lightleaking from said upper surface in the direction within said angle of 30degrees is not higher than ⅕ as high as said largest intensity of thelight outputted from said lower surface; and said light pipe has a hardcoat layer on said upper surface so that light incident on said lowersurface is transmitted out from said upper surface.

Further, according to the present invention, there is provided a surfacelight source device in which a light source is disposed on the incidentside surface of the above-mentioned light pipe, and there is provided areflection type liquid-crystal display device in which a liquid-crystalcell provided with a reflection layer is disposed on the lower surfaceside of the above-mentioned surface light source.

The operation and effect of the present invention are based oncharacteristic given to the light pipe. That is, while the inventors ofthe present invention have made eagerly investigation and investigationto overcome the aforementioned problems, the following fact has beenfound. In the background-art light pipe having diffusing dots or fineirregularities as a light output means, transmission light incident onthe side surface is scattered by the light output means of the lightpipe 18 so as to diverge in almost all directions as shown in FIGS. 9and 10. The output light α₁ from the lower surface and the leaking lightβ₃ from the upper surface exhibit the largest strengths B and b in adirection θ₄ at an angle of about 60 degrees to a normal line (frontaldirection) H with respect to the lower surface because of the scatteringcharacteristic of the light output means. Because the strengths areapproximately equal to each other, the quantity of light is short in adirection effective for visibility, especially in a viewing angle rangeof from upward 15 degrees to downward 30 degrees vertically and fromleftward 30 degrees to rightward 30 degrees horizontally on the basis ofthe normal line. As a result, brightness of display is short in theviewing angle range. Further, because the output light α₂ from the lowersurface to form a display image overlaps the leaking light β₄ from theupper surface, contrast is short. In the off-state of lighting, contrastis short because the display image is white-faded by the scattered lightγ₂ and the display image is disordered remarkably because two kinds ofdisplay light γ₁ and γ₃ are mixed with each other by scattering owing tothe light pipe.

The following fact has been further found. That is, also in the lightpipe having the prism type light output means according toJP-A-62-73206, like the aforementioned light pipe, a large amount oflight leaks from the upper surface. The leaking light overlaps theoutput light from the lower surface for forming the display image sothat contrast is lowered. Moreover, a large amount of light outputs at alarge output angle but the quantity of light in a direction effectivefor visibility is short to thereby lower brightness of display to causea problem in lowering of display quality.

Therefore, in order to form a bright and clear display image, the lightpipe needs to be provided as a light pipe capable of outputting lightincident on the side surface from the lower surface with gooddirectivity, especially with good light-condensing characteristic at anangle θ₃ as near to a direction of a normal line H with respect to thelower surface as possible as shown in FIG. 4 and particularly in theaforementioned viewing angle range. In a reflection type liquid-crystaldisplay device, an attempt to achieve uniformity and clarity of displayis generally made through a rough surface type reflection layer with amean diffusion angle in a range of from about 5 to about 15 degrees.Hence, if the quantity of light incident on the reflection layer at alarge angle to the reflection layer is large (B in FIG. 9, α₁ in FIG.10), the quantity of light effective for visibility is reduced tothereby make bright display difficult. Moreover, invisibility at such alarge angle, reversion of display is apt to occur. In electric fieldbirefringence type display, a problem in occurrence of a large colorchange etc. is apt to arise.

For improvement of contrast, the leaking light a from the upper surfaceneeds to be prevented as sufficiently from overlapping the output lightA from the lower surface to form a display image as possible, as shownin FIG. 4, especially needs to be prevented as sufficiently fromoverlapping the output light A in the aforementioned viewing angle rangeas possible. In the reflection type liquid-crystal display device,overlapping of the leaking light and the display image has largeinfluence on the contrast ratio because the contrast ratio is generallyin a range of from 1:5 to 1:20.

For prevention of disorder of the display image, light transmitted fromthe upper surface to the lower surface and from the lower surface to theupper surface needs to be prevented as sufficiently from being scatteredas possible. A front-lighting unit provided in the reflection typeliquid-crystal display device is an auxiliary light source for makingvisual recognition in a dark place possible. Hence, the front-lightingunit is adapted to visual recognition using external light such asindoor light or natural light for the original purpose of reduction ofconsumed electric power. Hence, if incidence of external light isblocked by the light pipe in the original condition that thefront-lighting unit is switched off, display becomes dark. If the lightis scattered by the light pipe in this condition, lowering of contrastis caused by white-fading of a surface or disorder of a display image iscaused by mixture of the display image, or the like.

In addition, from the point of view of preservation of theaforementioned light output characteristic and display quality for along term, the light pipe, especially the light output means formed onthe upper surface of the light pipe, needs to be prevented from beinginjured and abraded by wiping-out, or the like. The pixel pitch of theliquid-crystal display device is generally in a range of from 100 to 300μm. Hence, the influence of the light pipe on transmitted light needs tobe suppressed as sufficiently as possible so that information with apitch of about 100 μm can be recognized visually clearly. At the sametime, moire caused by interference with pixels needs to be alsosuppressed to obtain good display quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view for explaining a light pipe according tothe present invention;

FIGS. 2A through 2C are side views for explaining another light pipeaccording to the present invention;

FIGS. 3A and 3B are side views for explaining prism-like irregularities;

FIG. 4 is a view for explaining output characteristic according to anembodiment;

FIG. 5 is a side sectional view of a surface light source device;

FIG. 6 is a side sectional view of a reflection type liquid-crystaldisplay device;

FIG. 7 is a side sectional view of another reflection typeliquid-crystal display device;

FIG. 8 is a view for explaining a display image according to theembodiment;

FIG. 9 is a view for explaining output characteristic according to abackground-art example; and

FIG. 10 is a view for explaining a display image according to thebackground-art example.

BEST MODE FOR CARRYING OUT THE INVENTION

A light pipe according to the present invention is configured asfollows. Light incident on its incident side surface is outputted fromits lower surface through a light output means formed on its uppersurface. The direction of the largest intensity of the light outputtedfrom the lower surface is within an angle of 30 degrees to a normal linewith respect to a reference plane of the lower surface. The largestintensity of light leaking from the upper surface in the directionwithin the angle of 30 degrees is not higher than ⅕ as high as thelargest intensity of the light outputted from the lower surface. Thelight pipe has a hard coat layer on its upper surface so that lightincident on the hard coat layer from the lower surface is transmittedfrom the upper surface.

Examples of the light pipe according to the present invention are shownin FIG. 1 and FIGS. 2A to 2C. The reference numeral 1 designates a lightpipe; 11, 16 or 17, an upper surface having a light output means formedthereon; 12, a lower surface serving as the light output side; 13, anincident side surface; 14, a lateral side surface; and 15, an endopposite to the incident side surface. Further, the reference numeral 2designates a hard coat layer provided on the upper surface.

The light pipe according to the present invention is provided so thatlight incident on the incident side surface is outputted from the lowersurface through the light output means formed on the upper surface.Generally, the light pipe is made of a plate-like material having anupper surface, a lower surface opposite to the upper surface, and anincident side surface constituted by a side surface between the upperand lower surfaces. The plate-like material may be a plate of uniformthickness, or the like. Preferably, the plate-like material is providedas a plate in which the thickness of the end 15 opposite to the incidentside surface 13 is smaller than that of the incident side surface,especially the thickness of the opposite end 15 is not larger than 50%as large as the thickness of the incident side surface.

Thinning the opposite end permits light incident on the incident sidesurface as shown by the big arrow in FIGS. 3 and 4 to enter efficientlythe light output means formed on the upper surface until the lightreaches the opposite end. Hence, there is an advantage in that theincident light is outputted from the lower surface through reflection,or the like, so that the incident light can be supplied to a targetsurface efficiently. There is further an advantage in that the weight ofthe light pipe can be lightened. Incidentally, when the upper surface isprovided as a linear surface as shown in FIG. 2A, the weight of thelight pipe can be reduced to about 75% compared with the weight of alight pipe of uniform thickness.

Any light pipe may be used so long as the light pipe can exhibit theaforementioned output characteristic. Hence, the light output meansprovided on the upper surface of the plate-like material may be made ofa suitable material exhibiting such characteristic. The preferred fromthe point of view of the degree of achievement of the aforementionedcharacteristic, etc. is a light output means constituted by prism-likeirregularities.

The prism-like irregularities may be formed from convex or concaveportions each having equal-angle surfaces. From the point of view oflight utilizing efficiency, etc., the prism-like irregularities ispreferably formed from convex or concave portions each having a shortside surface and a long side surface. Examples of the prism-likeirregularities are shown in FIGS. 3A and 3B. The reference numeral 2 adesignates a convex portion; 2 b, a concave portion; 21 or 23, a shortside surface; and 22 or 24, a long side surface. Incidentally, thedistinction between the convex portions and the concave portions isdetermined by referring to a straight line connecting intersectionpoints between the short and long side surfaces and a surface forforming the short and long side surfaces, that is, the distinction isdetermined on the basis of the fact as to whether intersection points(vertices) between the short side surfaces and the long side surfacesare protruded or sunken from the straight line.

That is, in the case based on the example shown in FIG. 3, a straightline 20 expressed by a virtual line connecting intersection pointsbetween the short and long side surfaces (21 and 22, or 23 and 24) forforming the convex or concave portions 2 a or 2 b and a surface forforming the short and long side surfaces is referred to so that thedistinction is determined on the basis of the fact as to whetherintersection points (vertices) between the short and long side surfacesare protruded or sunken from the straight line 20.

The light pipe according to the present invention is configured so thatthe direction θ₃ of the largest intensity A of the light outputted fromthe lower surface 12 with respect to light (big arrow) incident on theincident side surface 13 as shown in FIG. 4 is within 30 degrees to anormal line H with respect to a reference plane of the lower surface andthat the largest intensity of light leaking from the upper surface inthe direction within 30 degrees is not larger than ⅕ as large as thelargest intensity A at the lower surface.

The light leaking from the upper surface is apt to overlap the reflectedlight, through a reflection layer, of the light exhibiting the largestintensity A. When the ratio of the largest intensity of the lightleaking from the upper surface to the largest intensity of the lightoutputted from the lower surface is high, the strength of a displayimage is apt to be relatively reduced so that contrast is apt to belowered.

The light pipe preferred from the point of view of improvement ofdisplay quality such as brightness and contrast in the case where thelight pipe is used for forming a reflection type liquid-crystal displaydevice is configured so that the direction θ₃ is within 28 degrees,especially within 25 degrees, more especially within 20 degrees in aplane (section in FIG. 4) perpendicular to both reference planes of theincident side surface 13 and the lower surface 12 as shown in FIG. 4.

In addition, the preferred light pipe is configured so that theintensity of light a leaking from the upper surface 11 at the same angleθ₃ as the direction of the largest intensity A in the case where theincident side surface 13 side is set as a negative direction withreference to the normal line H takes a value which is as small aspossible and which is not larger than {fraction (1/10)}, especially notlarger than {fraction (1/15)}, more especially not larger than {fraction(1/20)} as large as the largest intensity A. Because the leaking light aoverlaps the positive reflecting direction of the light exhibiting thelargest intensity A, the strength of the display image is relativelyreduced to thereby lower contrast when the value of the ratio a/A islarge.

The light output means preferred from the point of view of achievementof the direction of the largest intensity and the ratio of the leakinglight intensity to the largest intensity, etc. is constituted by arepetitive structure of arrangement of prism-like irregularities (2 a or2 b) each having a short side surface (θ₁) with an inclination angle ina range of from 30 to 40 degrees and a long side surface (θ₂) with aninclination angle in a range of from 0 to 10 degrees to the referenceplane of the lower surface 12 as shown in FIG. 3.

In the above description, each short side surface 21 or 23 formed as aslope inclined down from the incident side surface (13) side toward theopposite end (15) side has a role of reflecting a light part incident onthe short side surface among the light incident on the side surface tosupply the light part to the lower surface (light output surface). Inthis case, when the inclination angle θ₁ of the short side surface isset to be in a range of from 30 to 45 degrees, the short side surfacecan reflect the transmission light so efficiently as to be perpendicularto the lower surface as shown by the polygonal-line arrow in FIG. 3. Asa result, the light outputted from the lower surface can be obtained aslight exhibiting the direction θ₃ of the largest intensity A within 30degrees to the normal line H with respect to the lower surface as shownin FIG. 4.

The preferred inclination angle θ₁ of the short side surface from thepoint of view of the aforementioned performance such as suppression ofleaking light, suppression of visibility disturbance owing to theleaking light, etc. is in a range of from 32 to 43 degrees, especiallyin a range of from 35 to 42 degrees. Incidentally, if the inclinationangle θ₁ of the short side surface is smaller than 30 degrees, thedirection of the largest intensity of the light outputted from the lowersurface forms a large angle to the normal line so that brightness is aptto be lowered because of reduction of the quantity of light allowed tobe used for visual recognition. If the inclination angle θ₁ of the shortside surface is larger than 45 degrees, the quantity of light leakingfrom the upper surface is apt to increase.

On the other hand, each long side surface has the double purpose ofreflecting the transmission light incident on the long side surface tosupply the transmission light to a corresponding short side surface andtransmitting a display image from a liquid-crystal cell in the casewhere the light pipe is used for forming a reflection typeliquid-crystal display device. From this point of view, the inclinationangle θ₂ Of the long side surface to the reference plane (12) of thelower surface is preferably set to be in a range of from 0 to 10degrees.

From the above description, the transmission light with an angle largerthan the inclination angle θ₂ is incident on the long side surface 22 or24 and reflected as shown by the polygonal-line arrow in FIG. 3. In thiscase, the light is reflected to be more parallel to the lower surface 12on the basis of the inclination angle of the long side surface. Thereflected light enters the short side surface 21 or 23 and is reflectedby the short side surface. As a result, the light exits from the lowersurface 12 while the light is converged by the aforementionedparalleling operation.

As a result of the above description, the transmission light incident onthe short side surface through incidence on the long side surface andreflection at the long side surface in addition to the transmissionlight directly incident on the short side surface can be supplied to thelower surface by reflection through the short side surface. Improvementof light utilizing efficiency can be attained correspondingly. At thesame time, the incidence angle of light incident on the short sidesurface through reflection at the long side surface can be keptconstant. As a result, variations of the reflection angle can besuppressed, so that collimation and condensation of the light outputtedfrom the short side surface can be attained. Hence, directivity can begiven to the output light by adjustment of the respective inclinationangles of the short and long side surfaces. Hence, light can beoutputted at an angle perpendicular to the lower surface or at an anglenear to the perpendicular angle.

If the inclination angle θ₂ of the long side surface is 0 degrees, theeffect of collimating the transmission light is short. If theinclination angle is larger than 10 degrees, the rate of incidence onthe long side surfaces is lowered so that light emission is apt to bemade ununiform because of shortage of light supplied to the opposite endside. With respect to the sectional shape of the light pipe, it becomesdifficult to reduce the opposite end side thickness. Hence, the quantityof light incident on the prism-like irregularities is reduced so thatlight emitting efficiency is apt to be lowered. The inclination angle θ₂of the long side surfaces preferred from the point of view-of theaforementioned performance such as condensation of output light,suppression of leaking light, etc., based on collimation of transmissionlight is not larger than 8 degrees, especially not larger than 5degrees.

The long side surfaces preferred from the point of view of visualrecognition, or the like, of a display image through the long sidesurfaces of the light pipe are formed so that the angle differencesbetween their inclination angles θ₂ are set to be within 5 degrees,especially within 4 degrees, more especially within 3 degrees as a wholeof the light pipe and that the difference between inclination angles θ₂of adjacent long side surfaces is set to be within 1 degree, especiallywithin 0.3 degrees, more especially within 0.1 degrees.

In this manner, the influence of the difference between inclinationangles θ₂ of the long side surfaces permeating light on the displayimage can be suppressed. If deflection at a transmission angle of eachlong side surface varies widely in accordance with the place, thedisplay image is apt to be unnatural. Particularly if the difference indeflection between transmission images near to adjacent pixels is large,the display image is apt to be remarkably unnatural.

The aforementioned angle difference between inclination angles θ₂ holdson the premise that the inclination angles θ₂ of the long side surfacesare in a range of from 0 to 10 degrees. That is, the aforementionedangle difference holds on the premise that the inclination angles areset as such small angles θ₂ in an allowable range in order to suppressdeflection of the display image owing to refraction at the time oftransmission of light through the long side surfaces. This is for thepurpose of setting the point of observation in a neighborhood of avertical direction to prevent the optimum visibility direction of theoptimized liquid-crystal display device from being changed.

If the display image is deflected, the optimum visibility direction isshifted from the neighborhood of the vertical direction. Moreover, ifthe deflection of the display image is large, the optimum visibilitydirection comes close to the direction of emission of light leaking fromthe upper surface of the light pipe so that the leaking light may be aptto exert influence such as lowering of contrast on the optimumvisibility direction. Incidentally, the condition that the inclinationangle θ₂ of each long side surface is in a range of from 0 to 10 degreesincludes the influence of diffusion of transmitted light, or the like,is set to be ignorable.

The preferred from the point of view of obtaining a bright display imageis a light pipe excellent in efficiency of incidence of external lightand excellent in transmittance or output efficiency of an imagedisplayed by the liquid-crystal cell. From this point of view, the lightpipe preferably has prism-like irregularities so that the projected areaof the long side surfaces on the reference plane of the lower surface isnot smaller than 5 times, especially not smaller than 10 times, moreespecially not smaller than 15 times as large as the projected area ofthe short side surfaces on the reference plane. Hence, a large part ofthe image displayed by the liquid-crystal cell can be transmittedthrough the long side surfaces.

Incidentally, in the case where the image displayed by theliquid-crystal cell is transmitted, the display image incident on theshort side surfaces has little influence on the display image throughthe long side surfaces because the display image is reflected toward theincident side surface side so as not to exit from the upper surface oris deflected in a largely different direction on the end side oppositeto the display image transmitted through the long side surfaces withreference to the normal line with respect to the lower surface. Hence,the preferred from this point of view are short side surfaces which arenot localized with respect to the pixels of the liquid-crystal cell.Incidentally, in an extreme situation, if the short side surfacesoverlap the whole surface of the pixels, the display image can hardly bevisually recognized in the neighborhood of the vertical directionthrough the long side surfaces.

Hence, from the point of view of prevention of unnatural display owingto shortage of transmission of display light, and so on, the area ofoverlap of the short side surfaces with the pixels is preferably reducedto secure sufficient transmittance of light through the long sidesurfaces. From the aforementioned point of view, the short side surfacesare preferably formed so that the projected width of the short sidesurfaces on the reference plane of the lower surface is not larger than40 μm, especially in a range of from 1 to 20 μm, more especially in arange of from 5 to 15 μm considering that the pixel pitch of theliquid-crystal cell is generally in a range of from 100 to 300 μm.

It is also preferable from the aforementioned point of view that thedistance between adjacent short side surfaces is large. On the otherhand, if the distance is too large, illumination in a lighting mode maybecome sparse to cause still unnatural display because the short sidesurfaces form a functional portion for substantially outputting lightincident on the side surface. Considering these, the repetitive pitch Pof a prism-like irregularity 2 a or 2 b is preferably set to be in arange of from 50 μm to 1.5 mm as shown in FIG. 3. Incidentally, thepitch may be a regular pitch or may be an irregular pitch such as arandom pitch, a combination of a predetermined number of pitch units atrandom or regularly, etc.

When the light output means is constituted by prism-like irregularities,the light output means may interfere with the pixels of theliquid-crystal cell to cause moire. Although prevention of moire can beperformed by adjustment of the pitch of the prism-like irregularities,the pitch of the prism-like irregularities is limited to the preferredrange as described above. Hence, measures against moire generated in thepitch range are a subject of discussion.

In the present invention, there is preferably used a method in which theprism-like irregularities are formed to be inclined with respect to thereference plane of the incident side surface so that the prism-likeirregularities are arranged to cross the pixels to thereby preventmoire. In this case, if the inclination angles are too large, deflectionoccurs in reflection through the short side surfaces. Hence, largepolarization occurs in the direction of output light. Hence, anisotropyof emission intensity in the light-transmitting direction of the lightpipe becomes large. Hence, light-utilizing efficiency is also reduced.This is apt to be a cause of lowering of display quality.

From the aforementioned point of view, the direction of arrangement ofthe prism-like irregularities with respect to the reference plane of theincident side surface, that is, the inclination angle in the ridgelinedirection of the prism-like irregularities is preferably set to bewithin ±30 degrees, especially within ±25 degrees, more especiallywithin ±20 degrees. Incidentally, the symbol “±” means the direction ofinclination with reference to the incident side surface. When moire canbe ignored, it is more preferable that the direction of arrangement ofthe prism-like irregularities is set to be as parallel to the incidentside surface as possible.

The light pipe can be provided as a suitable form as described above.Also in the case where the light pipe is shaped like a wedge, or thelike, the shape can be determined suitably. The surface shape of thelight pipe can be set as a suitable surface shape such as a linearsurface 11 as shown in FIG. 2A, a curved surface 16 or 17 as shown inFIG. 2B or FIG. 2C, or the like.

Also the prism-like irregularities constituting the light output meansneed not be formed from linear surfaces 21, 22, 23 and 24 shown in FIG.3. The surface shape of each of the prism-like irregularities may beprovided as a suitable surface shape including a bent surface, a curvedsurface, etc. Further, the prism-like irregularities may be constitutedby a combination of irregularities different in shape, etc. in additionto pitch. Further, the prism-like irregularities may be formed as aseries of convex or concave portions arranged continuously in terms ofridgeline or may be formed as convex or concave portions arrangeddiscontinuously at predetermined intervals intermittently in terms ofridgeline direction.

As shown in the drawings, a hard coat layer 2 is provided on the uppersurface of the light pipe. The hard coat layer is provided for thepurpose of keeping on the aforementioned light output characteristic fora long term by preventing the light output means formed on the uppersurface from being injured or abraded by wiping-out, or the like.Therefore, the hard coat layer can be formed as a suitable transparenthard film by a suitable forming method in accordance with the backgroundart, such as a vapor deposition film made of a glass material such assilicon dioxide, zirconia etc., especially made of a hard glassmaterial, a coating film of a hard polymer such as a silicone polymer, afluorine polymer, etc., and so on. The material of the hard coat layeris not particularly limited.

Also the thickness of the hard coat layer is not particularly limited.Generally, from the point of view of practical use, etc., the thicknessis set to be not larger than 3 μm, especially not larger than 2 μm, moreespecially in a range of from 0.1 to 1 μm. When the light output meansis constituted by prism-like irregularities, it is preferable that thehard coat layer has the aforementioned thickness, from the point of viewof the shape characteristic, etc. thereof.

The shape of the lower surface of the light pipe and the shape of theincident side surface thereof are not particularly limited and may bedetermined suitably. Generally, the lower surface is formed as a flatsurface and the incident side surface is formed as a surfaceperpendicular to the lower surface. Incidentally, with respect to thelower surface, the light output means may interfere with reflectionpatterns of the light output means in the lower surface to thereby forminterference fringes in accordance with the direction of visualrecognition when viewed from the upper surface. Therefore, if necessary,the lower surface may be formed to have a fine irregular structure forthe purpose of preventing lowering of display quality from being causedby the moire phenomenon. The fine irregularities can be formed by asuitable method in accordance with the background-art diffusing layer,such as a method of roughening the lower surface of the light pipe, amethod of providing a transparent fine particle-containing resin layeror diffusing sheet to the lower surface of the light pipe, and so on.

Also with respect to the incident side surface, for example, it may beshaped like a curved concave in accordance with the outer circumferenceof the light source, or the like, to attain improvement of lightincidence efficiency. Further, the incident side surface may be formedas an incident side surface structure having an introducer portioninterposed between the light source and the incident side surface. Theshape of the introducer portion can be formed as a suitable shape inaccordance with the shape of the light source.

In the above description, the light pipe preferred from the point ofview of prevention of lowering of visibility characteristic caused bydisorder of a display image owing to scattering, achievement of a cleardisplay image, etc. is a light pipe in which the total light-raystransmittance of incident light in directions of the upper and lowersurfaces, especially the total light-rays transmittance of perpendicularincident light from the lower surface to the upper surface is not lowerthan 90%, especially not lower than 92%, more especially not lower than95% and in which haze is not higher than 45%, especially not higher than30%, more especially not higher than 20%.

The light pipe may be made of a suitable material exhibitingtransparency in accordance with the wavelength range of the lightsource. Incidentally, when the wavelength range is a visible lightrange, transparent resin, as represented by acrylic resin, polycarbonateresin, epoxy resin, or the like, glass, etc. may be used. A light pipemade of a material not exhibiting birefringence or small inbirefringence is used preferably.

The light pipe may be also formed by a cutting method and may be formedby a suitable method. Examples of the producing method preferred fromthe point of view of mass-production, etc. are a method of transferringa shape by pressing a thermoplastic resin against a mold capable offorming a predetermined shape under heating, a method of filling a moldcapable of forming a predetermined shape with a hot-melted thermoplasticresin or with a resin fluidized through heat or solvent, a method ofperforming polymerization by filling a mold capable of forming apredetermined shape with a liquid resin polymerizable by heat,ultraviolet light, radiation, or the like, or by casting the liquidresin into the mold, and so on.

Incidentally, the light pipe according to the present invention may beformed as a laminate of parts made of the same kind or different kindsof materials such as a laminate of a sheet having a light output meansconstituted by prism-like irregularities and a light guide portion beingin charge of light transmission and adhesively bonded to the sheet. Thatis, the light pipe need not be formed as an integrated monolayerstructure made of one kind of material. The thickness of the light pipecan be determined suitably in accordance with the size of the lightpipe, the size of the light source, etc. in accordance with the purposeof use. The general thickness in the case where the light pipe is usedfor forming a reflection type liquid-crystal display device is notlarger than 20 mm, especially in a range of from 0.1 to 10 mm, moreespecially in a range of from 0.5 to 8 mm in terms of the thickness ofthe incident side surface.

In the light pipe according to the present invention, light incident onthe upper and lower surfaces is transmitted the lower or upper surfacessufficiently. Various devices such as a surface light source deviceutilizing such light emitted from the light source so efficiently as tobe excellent in brightness, a reflection type liquid-crystal displaydevice which is bright and easy to watch and which is excellent in powersaving, and so on, can be formed by use of the light pipe becauseaccurately collimated light can be output in a direction excellent inperpendicular characteristic advantageous in visual recognition.

FIG. 5 shows a surface light source device 3 having the light pipe 1according to the present invention. For example, the surface lightsource device can be formed by arrangement of a light source 31 on theincident side surface of the light pipe 1 as shown in FIG. 5. Thesurface light source device can be preferably used as a side-light typefront-lighting unit, or the like.

A suitable light source may be used as the light source disposed on theincident side surface of the light pipe. Generally, for example, alinear light source such as a (cold-or hot-) cathode tube, a point lightsource such as a light-emitting diode, an array light source havingarrangement of linear or point light sources in a line or plane, a lightsource using a device for converting a point light source into a stateof linear emission of light at regular or irregular intervals, and soon, may be used preferably. From the point of view of power saving,durability, etc., the cold-cathode tube is especially preferably used.

The surface light source device may be formed as a combination body inwhich suitable auxiliary means such as a light-source holder 32 forenclosing the light source to lead light scattered from the light source31 to the incident side surface of the light pipe 1 as shown in FIG. 5,a diffusing layer 4 disposed on the lower surface of the light pipe toobtain uniform surface light emission as shown in FIG. 6, and so on, aredisposed as occasion demands.

A resin sheet coated with a high-reflectance metal thin film or a sheetof metal foil, or the like, is generally used as the light sourceholder. When the light source holder is bonded to an end portion of thelight pipe through an adhesive agent, or the like, the formation of thelight output means in the adhesive portion may be omitted.

The diffusing layer is disposed on the light output surface of thesurface light source device, that is, on the lower surface 12 of thelight pipe 1 in advance as occasion demands for the purpose ofuniformity of brightness by preventing brightness irregularity,reduction of moire by mixing adjacent light rays, etc. In the presentinvention, a diffusing layer having an arrow diffusing range ispreferably used from the point of view of preservation of directivity ofoutput light of the light pipe, light-utilizing efficiency, etc.

The diffusing layer can be formed by a suitable method, for example,such as a method of applying and hardening high-refractive-indextransparent particles dispersed into a low-refractive-index transparentresin in accordance with the fine irregularities of the lower surface, amethod of applying and hardening a transparent resin having air bubblesdispersed therein, a method of crazing a surface of a substrate byswelling out the surface through a solvent, a method of forming atransparent resin layer having an irregular rough surface or a methodusing a diffusing sheet formed in the same manner as described above.The method for forming the diffusing layer is not particularly limited.The aforementioned irregular rough surface may be formed by a suitablemethod such as a mechanical or/and chemical treatment method oftransferring a rough surface shape of a roll, a mold, or the like,subjected to a surface-roughening treatment onto a surface of asubstrate or a surface of a transparent resin coating layer provided onthe substrate.

As described above, the surface light source device according to thepresent invention provides light which is excellent in light-utilizingefficiency and which is bright and excellent in perpendicularcharacteristic. The surface light source device is also easy to increasethe area thereof. Therefore, the surface light source device can bepreferably applied to various devices. For example, the surface lightsource device can be used as a front-lighting system for a reflectiontype liquid-crystal display device. Hence, a power-saving reflectiontype liquid-crystal display device which is bright and easy to watch, orthe like, can be obtained.

FIGS. 6 and 7 show examples of the reflection type liquid-crystaldisplay device using the surface light source device 3 according to thepresent invention as a front-lighting system. The reference numerals 5and 51 designate polarizing plates; 6, a liquid-crystal cell; 61 and 63,cell substrates; 62, a liquid-crystal layer; and 7 and 64, reflectionlayers. The reflection type liquid-crystal display device can be formedby arrangement of the liquid-crystal cell 6 having a reflection layer 7or 64 on the light output side of the surface light source device asshown in the drawings, that is, on the lower surface side of the lightpipe 1 in the surface light source device.

The reflection type liquid-crystal display device is generally formed byassembling constituent parts such as a liquid-crystal cell having atransparent electrode serving as a liquid-crystal shutter, a drive unitattached to the liquid-crystal cell, a polarizing plate, afront-lighting unit, a reflection layer and a compensatory retarderplate as occasion demands. In the present invention, the reflection typeliquid-crystal display device can be formed in accordance with thebackground art as shown in the drawings without particular limitationexcept that the aforementioned surface light source device is used.Incidentally, in the example shown in FIG. 6, the transparent electrodeis not shown.

Hence, the liquid-crystal cell to be used is not particularly limited.For example, there may be used a suitable liquid-crystal cell such as aTN or STN liquid-crystal cell, a perpendicularly oriented or HAN cell, atwisted or non-twisted cell such as an OCB cell or a guest-host orferroelectric liquid-crystal cell in terms of the orientation form ofthe liquid-crystal cell. The liquid-crystal drive method is notparticularly limited. For example, there may be used a suitable drivemethod such as an active matrix method or a passive matrix method.

The arrangement of the reflection layer 7 or 64 is essential to thereflection type liquid-crystal display device. The position ofarrangement of the reflection layer, however, may be provided outsidethe liquid-crystal cell 6 as shown in FIG. 6, or inside theliquid-crystal cell 6 as shown in FIG. 7. The reflection layer may beformed as a suitable reflection layer in accordance with the backgroundart such as a coating layer of a binder resin containinghigh-reflectance metal powder such as aluminum, silver, gold, copper,chromium, etc., an attachment layer of a metal thin film by a vapordeposition method, or the like, or a reflection sheet or a sheet ofmetal foil constituted by the coating or attachment layer supported by asubstrate.

Incidentally, when the reflection layer 64 is provided inside theliquid-crystal cell 6 as shown in FIG. 7, a reflection layer formed-by amethod of forming an electrode pattern from a highly electricallyconductive material such as the aforementioned high-reflectance metal orby a method of forming a transparent electrically conductive film, forexample, from the transparent electrode-forming material on atransparent electrode pattern is preferably used as the reflectionlayer.

A suitable material can be used as the polarizing plate. From the pointof view of obtaining good contrast-ratio display based on incidence ofhigh-grade linearly polarized light, etc., a material high in the degreeof polarization such as an absorption type linearly polarizing elementof iodine or dye can be used preferably.

Incidentally, for the formation of the reflection type liquid-crystaldisplay device, suitable optical devices such as a diffusing plate, ananti-glare layer, an anti-reflection film and a protective layer to beprovided on the visual side polarizing plate or a compensatory retarderplate to be provided between the liquid-crystal cell and the polarizingplate may be disposed suitably.

The aforementioned compensatory retarder plate is provided for thepurpose of compensating for wavelength dependence of birefringence toattain improvement of visibility, etc. In the present invention, one ortwo compensatory retarder plates can be disposed between the visual sidepolarizing plate and the liquid-crystal cell or/and between the backside polarizing plate and the liquid-crystal cell as occasion demands. Asuitable member may be used as the compensatory retarder plate inaccordance with the wavelength range. The compensatory retarder platemay be formed as one layer or as a multilayer laminate constituted-bytwo or more phase difference layers.

Visual recognition of the reflection type liquid-crystal display deviceaccording to the present invention is performed through lighttransmitted through the surface light source device, especially lighttransmitted through the long side surfaces of the light pipe in thesurface light source device. FIG. 8 shows, by way of example, a visualrecognition state in the case where the reflection layer 64 is providedinside the liquid-crystal cell. In FIG. 8, when the surface light sourcedevice is turned on, light α emitted from the lower surface of the lightpipe 1 is reflected through the reflection layer 64 via the polarizingplate 5, the liquid-crystal layer 62, etc. Hence, the light α reachesthe light pipe 1 reversely via the liquid-crystal layer, the polarizingplate, etc. As a result, a display image (α) transmitted through thelong side surfaces 22 is viewed.

In the aforementioned case, in the present invention, strong leakinglight 81 exits in a direction largely angularly divergent from a frontaldirection perpendicular to the liquid-crystal cell whereas leaking lightβ₂ exiting in the frontal direction is weak. Hence, a display imageexcellent in display quality in a neighborhood of the frontal directionthrough the long side surfaces can be viewed.

On the other hand, also when the surface light source device is turnedoff so that external light is used, light γ input from the long sidesurfaces 22 in the upper surface of the light pipe 1 is transmittedthrough the polarizing plate, the liquid-crystal layer, the reflectionlayer, etc. in the same manner as described above. Hence, the light γreaches the light pipe 1 in the reverse course. As a result, a displayimage (γ) transmitted through the long side surfaces can be viewed in astate in which the display image is so excellent in display quality thatdisorder, or the like, owing to the light pipe is short in aneighborhood of the frontal direction.

According to the present invention, optical devices or parts for formingthe surface light source device or the liquid-crystal display device,such as a light pipe, a diffusing layer, a liquid-crystal cell, apolarizing plate, etc., may be wholly or partly integrally laminated andfixed with one another or may be disposed in a separable state. From thepoint of view of prevention of lowering of contrast owing to suppressionof interface reflection, etc., it is preferable that the optical devicesor parts are fixed and that at least the lower surface of the light pipein the surface light source device is fixed closely to the upper surfaceof the liquid-crystal cell.

A suitable transparent adhesive agent such as a tackifier can be usedfor the aforementioned closely fixing process. The aforementionedtransparent particles, or the like, may be contained in the transparentadhesive layer so that the adhesive layer can be used as an adhesivelayer exhibiting a diffusing function.

EXAMPLE 1

An upper surface of a transparent plate of polymethyl methacrylate(PMMA) was cut by a diamond bite to thereby obtain a plate-like material80 mm wide and 130 mm deep. The plate-like material had an incident sidesurface 2 mm thick, an opposite end surface 0.8 mm thick, a lowersurface (output surface) provided as a flat surface, an upper surfaceprovided as a nearly planar curved surface (FIG. 2B) protruded upwardfrom the incident side surface to the opposite end surface, andprism-like irregularities arranged at intervals of a pitch of 390 μm onthe curved surface so as to be parallel to the incident side surface.Each of the prism-like irregularities had a short side surface, and along side surface. The respective inclination angles of the short sidesurfaces changed in a range of from 36.5 to 39 degrees. The respectiveinclination angles of the long side surfaces changed in a range of from1.1 to 1.5 degrees. The inclination angle change between adjacent longside surfaces was within 0.1 degrees. The projected width of the shortside surfaces on the lower surface was in a range of from 10 to 21 μm.The projected area ratio of the long side surfaces to the short sidesurfaces on the lower surface was not lower than 17/1. A hard coat layerof SiO₂ 0.3 m thick was provided on the upper surface of the plate-likematerial by a vacuum vapor deposition method.

A cold-cathode tube with a diameter of 2.4 mm was disposed on theincident side surface of the light pipe. An edge of the cold-cathodetube was enclosed in a light source holder made of a polyester filmcoated with silver by vapor deposition while the edge was brought intoclose contact with the upper and lower end surfaces of the light pipe.An inverter and a DC power supply were connected to the cold-cathodetube. Thus, a surface light source device was obtained. A monochromereflection type TN liquid-crystal cell having a reflection sheet similarto the light source holder on its back was disposed on the light outputside (the lower surface of the light pipe. Thus, a reflection typeliquid-crystal display device was obtained.

Comparative Example

A surface light source device and a reflection type liquid-crystaldisplay device were obtained in the same manner as in Example 1 exceptthat a light pipe having no hard coat layer on its upper surface wasused.

Evaluation Test

The following characteristics were examined upon the light pipe, thesurface light source device and the reflection type liquid-crystaldisplay device obtained in each of Example and Comparative Example.

Output Intensity

The angle characteristic of output intensity in the upper and lowersurfaces in the center portion of the light pipe was examined by abrightness meter (BM7, made by Topcon Corp.) in the condition that thesurface light source device was turned on. Measurement was performedwhile the angle was changed with reference to the direction of a normalline with respect to the lower surface in a plane perpendicular both tothe lower surface and to the incident side surface. To make themeasurement area constant, the measured value was multiplied by cosineof the measurement angle θ to thereby obtain output intensity at θ. Alsothe output direction of the maximum intensity was obtained. The maximumintensity and direction thereof in the lower surface and the outputintensity (corresponding output intensity) in the upper surface in amirror symmetric direction with reference to the maximum intensitydirection in the lower surface, a normal line and the lower surface wereshown in the following Table.

Solid 30-Degrees Light Quantity

A cylindrical jig having a hole with a diameter of 10 mm and an innersurface delustered with black was disposed on a side opposite to anilluminance meter so that the solid angle between the hole and alight-receiving surface of the illuminance meter became 30 degrees. Thequantity of light emitted within a solid angle of 30 degrees from theupper and lower surfaces of the surface light source device in alighting state was examined by use of the jig. The quantity of light andthe total light-rays transmittance and haze of the light pipe were shownin the following Table.

Frontal Luminance and Visibility

Frontal luminance and visibility in a while state were examined in adriving state of the reflection type liquid-crystal display device underthe condition that the surface light source device was turned on.Results thereof were also shown in the following Table. Incidentally,frontal luminance examined for reference in the same manner as describedabove in the case where the surface light source device was not disposedwas 28 cd/m².

Mar-proofness

In the condition that the surface of the light pipe in the reflectiontype liquid-crystal display device was rubbed with steel wool, frontalluminance and visibility were examined in the same manner as describedabove. Results thereof were shown in the following Table.

Comparative Example 1 Example Lower side maximum intensity angle(degree) 16 16 Lower side maximum intensity (cd/m²) 680 680 Upper sidecorresponding output 34 34 intensity (cd/m²) Upper side/lower sidecorresponding maximum 0.050 0.050 output intensity ratio Lower sidesolid 30-degrees 14.5 14.5 light quantity (LX) Upper side solid30-degrees 1.3 1.3 light quantity (LX) Total light-rays transmittance92.4 92.4 Haze (%) 7.4 7.4 Frontal luminance (cd/m²) 170 170 VisibilityGood Good Mar-proofness Frontal luminance (cd/m²) 170 143 VisibilityGood White fading

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, there can beobtained a light pipe in which light outputted from its lower surface isexcellent in directivity in a vertical direction and in which lightleaking from its upper surface hardly overlaps a display image. The useof the light pipe makes it possible to obtain a surface light sourcedevice excellent in light utilizing efficiency. Moreover, it is possibleto obtain a reflection type liquid-crystal display device which isexcellent in contrast in visual recognition both at the time ofnon-lighting and at the time of lighting, excellent in brightness ofdisplay and excellent in clarity because of the light pipe's suppressingdisorder of a display image and which keeps up initial outputcharacteristic and display quality for a long term because a lightoutput means is hardly abraded or injured by wiping-out, or the like.

What is claimed is:
 1. A light pipe, comprising: an incident sidesurface on which light is incident; an upper surface on which a lightoutput means is formed; a lower surface from which the light incident onsaid incident side surface and through said output means is outputted;and wherein a direction of a largest intensity of the light outputtedfrom said lower surface is within an angle of 30 degrees to a normalline with respect to a reference plane of said lower surface; and alargest intensity of light leaking from said upper surface in adirection within said angle of 30 degrees is not higher than ⅕ as highas said largest intensity of the light outputted from said lowersurface; and said light pipe has a hard coat layer on said upper surfaceso that light incident on said lower surface is transmitted from saidupper surface.
 2. The surface light source device comprising a lightpipe according to claim 1, comprising a light source disposed on theincident side surface of said light pipe.
 3. The reflection-type liquidcrystal display device comprising a surface light source deviceaccording to claim 2, and comprising a liquid-crystal cell provided witha reflection layer and disposed on the lower surface side of saidsurface light source.
 4. A light pipe, comprising: an incident sidesurface on which light is incident; an upper surface on which a lightoutput means is formed; a lower surface from which the light incident onsaid incident side surface and through said output means is outputted;and wherein a direction of a largest intensity of the light outputtedfrom said lower surface is within an angle of 30 degrees to a normalline with respect to a reference plane of said lower surface; and alargest intensity of light leaking from said upper surface in adirection within said angle of 30 degrees is not higher than ⅕ as highas said largest intensity of the light outputted from said lowersurface; and said light pipe has a hard coat layer on said upper surfaceso that light incident on said lower surface is transmitted from saidupper surface; and wherein: said light output means is constituted by arepetitive structure in which prism-like irregularities each having ashort side surface and a long side surface are arranged at intervals ofa pitch in a range of from 50 μm to 1.5 mm; each of said short sidesurfaces is constituted by a slope inclined down from the incident sidesurface side to the opposite end side with an inclination angle in arange of from 30 to 45 degrees to said reference plane of said lowersurface and with a projected width of not larger than 40 μm on saidreference plane; and each of said long side surfaces is constituted by aslope which is inclined at an angle in an inclination angle range offrom 0 to 10 degrees to said reference plane so that the inclinationangle difference between said long side surfaces as a whole is within 5degrees, that the inclination angle difference between adjacent longside surfaces is within 1 degree and that the projected area of saidlong side surfaces on said reference plane is not smaller than 5 timesas large as the projected area of said short side surfaces on saidreference plane.
 5. A light pipe according to claim 4, wherein theridgeline direction of said prism-like irregularities is within an anglerange of ±30 degrees to a reference plane of said incident side surface.6. A surface light source device comprising a light pipe according toclaim 4, and comprising a light source disposed on the incident sidesurface of said light pipe.
 7. A reflection type liquid-crystal displaydevice comprising a surface light source device according to claim 4,and comprising a liquid-crystal cell provided with a reflection layerand disposed on the lower surface side of said surface light source. 8.The surface light source device comprising a light pipe according toclaim 5, comprising a light source disposed on the incident side surfaceof said light pipe.
 9. The reflection-type liquid crystal display devicecomprising a surface light source device according to claim 8, andcomprising a liquid-crystal cell provided with a reflection layer anddisposed on the lower surface side of said surface light source.
 10. Alight pipe, comprising: an incident side surface on which light isincident; an upper surface on which a light output means is formed; alower surface from which the light incident on said incident sidesurface and through said output means is outputted; wherein said lightpipe has a hard coat layer on said upper surface so that light incidenton said lower surface is transmitted from said upper surface; andwherein said light output means is constituted by a repetitive structurein which prism-like irregularities, each having a short side surface anda long side surface, are arranged at intervals of a pitch in a range offrom 50 μm to 1.5 mm; each of said short side surfaces is constituted bya slope inclined down from the incident side surface side to theopposite end side with an inclination angle in a range of from 30 to 45degrees to a reference plane of said lower surface and with a projectedwidth of not larger than 40 μm on said reference plane; and each of saidlong side surfaces is constituted by a slope which is inclined at anangle in an inclination angle range of from 0 to 10 degrees to saidreference plane so that the inclination angle difference between saidlong side surfaces as a whole is within 5 degrees, that the inclinationangle difference between adjacent long side surfaces is within 1 degreeand that the projected area of said long side surfaces on said referenceplane is not smaller than 5 times as large as the projected area of saidshort side surfaces on said reference plane.